riscv: use memcpy based uaccess for nommu again

This reverts commit adccfb1a

.

Now that the generic uaccess by mempcy code handles unaligned addresses
the generic code can be used for all RISC-V CPUs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>